While it's ideal to have both front/rear sight fixed, it's contra-indicated on a gun with a removeable barrel, especially since the best use of ANY peep sight, ghost ring included, is that they be located as close to the eye as possible (why tang sights are favored on some rifles).

When any peep sight is positioned too far away from the shooter's eye, as on the barrel, there's a decided tendency to look at the peep/aperture instead of thorugh it w/o actually seeing it, as is the case when it's closer to the eye.